Microflex circuits are widely used for applications such as integrated circuit (IC) packages, inkjet cartridges, hard disk drives (HDD), medical devices and other advanced flex circuit-based electrical connections. Such connections require electrical static discharge (ESD) protection during product manufacturing and assembly comprising insulating materials (plastic, polymers) and manual operations. ESD concerns are amplified in hard disk drive manufacturing environments and applications, especially for head gimbal assemblies (HGA) using giant magnetorestrictive read (GMR) sensors in the sliders. For current commercial hard disk drive (HDD) product platforms at 40 or 60 gigabit/in2 [(GB)/in2], electrostatic discharge protection is required to protect these sensitive read heads. ESD requirements are specified in terms of surface resistivity and tribocharge voltage control on all incoming components and nearby process/assembly workstations. The specifications from many HDD manufacturers indicated surface resistivities should preferably be controlled within a range of from about 104 ohms to about 108 ohms, and maximum surface voltage from tribocharging must be less than 10 Volts (V).
Based on latest HGA technology roadmaps, future HDD programs will be 80 to 100 GB/platter or higher, and will have stricter ESD requirements (tribocharge less than 5 V) for incoming components including flexible circuits for use in flex on suspension (FOS). Microactuator programs, complex slider structures and additional electrical connections and attachment on suspensions will require a good control of ESD as well. Damage caused by ESD accounts for a large portion of the HGA yield losses in the HDD industry, which has a significant impact on industry profits. Therefore, the ability to control ESD is very important to the HDD industry.
In U.S. Pat. No. 6,459,043 the application of a thin, narrow stripe of conductive composition was applied across at least two of the conductive elements of a flexible circuit to reduce the accumulation of static charge and balance the electrical potential across all of the conductive leads. This application also discloses that placing the conductive polymer layer beneath the protective covercoat layer allows the resulting FOS circuit to pass the HDD industry's strict ionic contamination testing.
However, it has now been discovered that a flexible circuit having a thin conductive polymer coating applied over a portion of a flexible circuit in the non-critical region reduces the measured surface voltage substantially.